Absorbent articles for personal hygiene, such as disposable diapers, feminine protection sanitary pads and adult incontinence undergarments, are designed to absorb and contain body exudates, in particular but not limited to urine. These absorbent articles usually comprise several layers having different functions, for example a topsheet, a backsheet and in-between an absorbent core, among other layers. The function of the absorbent core is to absorb and retain the exudates for a prolonged amount of time, for example overnight for a diaper, minimize re-wet to keep the wearer dry and avoid soiling of clothes or bed sheets.
The majority of currently marketed absorbent article comprise as absorbent material a blend of comminuted wood pulp with superabsorbent polymers (SAP) in particulate form, also called absorbent gelling materials (AGM), see for example U.S. Pat. No. 5,151,092 (Buell). Absorbent articles having a core consisting of essentially SAP as absorbent material (so called “airfelt-free” cores) have also been proposed but are less common than traditional mixed cores, see e.g. WO2008/155699 (Hundorf), WO95/11652 (Tanzer), WO2012/052172 (Van Malderen). Some absorbent cores have a profiled distribution of SAP towards the front of the absorbent core where more absorbent capacity is required because urine is typically delivered towards the front of the article.
It is known to provide a sub-layer, typically a non-woven, between the topsheet and the absorbent core. These sub-layers are designed to quickly acquire and/or distribute the fluid away from the topsheet and into the core. These sub-layers are sometimes called “wicking layer”, “surge layer”, “acquisition layer” or “distribution layer”. Articles having only one of these sub-layers are known. Articles having two sub-layers or more, in particular a first sub-layer having a high capillarity which pulls the fluid quickly away from the topsheet and a second sub-layer a larger void area to distribute the fluid over a large surface over the core, are also known. These sub-layers typically do not comprise superabsorbent articles material. In the following, the term “acquisition-distribution system” (“ADS”) will be used to designate the layer or combination of discrete layers (one, two, or more) present between the topsheet and the backsheet and providing these acquisition and/or distribution function, irrespective of the number of layers.
Acquisition-distribution systems comprising a single layer are disclosed for example in WO94/23761 (Payne), which discloses an acquisition layer comprising an homogeneous composition of hydrophilic fibrous material and a storage layer comprising a mixture of hydrophilic fibrous material and discrete particles of absorbent gelling material. The acquisition layer has an acquisition zone towards the front of the article of relatively lower average density and relatively lower average basis weight than a distribution zone towards the back of the article. Another example of ADS having a single layer can be found in U.S. Pat. Nos. 5,486,166 and 5,490,846 (Bishop).
US2008/0312621 and US2008/0312622 (Hundorf) describe a disposable absorbent article comprising a chassis including a topsheet and a backsheet, a substantially cellulose free absorbent core located between the topsheet and the backsheet and having a wearer facing side oriented toward a wearer when the article is being worn and an opposed garment facing side, and a “liquid acquisition system” comprising chemically cross-linked cellulosic fibers disposed between the liquid permeable topsheet and the wearer facing side of the absorbent core. The liquid acquisition system may also comprise an upper acquisition layer made of a latex bonded non-woven.
WO99/17679 (Everett) discloses an absorbent core having multiple absorbent layers, wherein the absorbent layers interact in such a manner which preferentially locates absorbed liquid in an appointed, high saturation wicking layer within the core. This document also disclose a so-called “surge management layer” located on an inwardly facing body side surface of the topsheet layer. As shown in the drawings, this surge management layer is placed towards the front of the article and has a uniform basis weight along its length.
Typically absorbent cores have a higher absorbency capacity towards the front of the article as the fluid insult typically happens towards the front of the article. Acquisition-distribution systems have also been typically placed towards the front of the article for the same reason. After analyzing several hundred returned used diapers, the inventors have however surprisingly found that while it is true that the majority of the fluid insult happens towards the front of the article, a non negligible amount of fluid is also acquired in the article further back, in particular at a distance of approximately one third from the back edge of the article. The inventors surprisingly determined that the amount of fluid acquired in the back of the diaper can be as low as 1/100 of the one acquired at the front, but sometimes as high as approximately half the amount acquired at the front in some special circumstances, such as high loads for baby girls laying on the back.
The inventors believe that at high loadings the fluid present towards the front of the article can saturate the absorbent material of the core, which slows its absorption. The inventors also believe that in some conditions, the fluid pooling between the skin and the topsheet can be lead by gravity towards the back of the absorbent article, especially if the wearer is sleeping on its back. This principle may be the cause of the surprising presence of high amount of fluid in the back of the diaper, which may cause of a non-negligible amount of diaper leakages during the night in the area of the back of the absorbent article. Although one solution to this problem would be to uniformly increase the basis weight of the existing acquisition-distribution system, this unfavorably increases production costs and may not prevent the fluid running at the surface of the wearer's skin.
The present inventors are proposing instead an improved construction for absorbent articles having absorbent cores with high concentration of SAP, which in particular may provide improvements in acquisition speed at high loading and/or during particular wearing conditions. The proposed ADS of the invention extends towards the back of the article at least to a certain point (A2) while at the same having a reduced basis weight towards the back of the article. The inventors have found that providing a reduced amount of acquisition-distribution material towards the back of the article at the point A2 was still sufficient to accommodate the lower amount of fluid expected in this part of the article.
In particular the inventors believe that the addition of a lower amount of acquisition-distribution material in the back of the article is sufficient to capture this lower amount of fluid reaching this area and improve the overall performance of the absorbent article. It is also believed that this new construction can improve the overall acquisition speed of the article without significantly increasing costs, even when the back of the core comprises relatively low amount of SAP. The inventors also believe that the acquisition-distribution material now placed towards the back of the article may be useful to acquire some of the fluid which first injured the article towards its front, and to later redistribute the fluid to the front of the core where the higher capacity is present. Additionally a lower basis weight for the acquisition distribution system at the back is believed to be beneficial to reduce the risk of saturating the back of the absorbent core, where capacity can be more limited than at the front.